JPH0358655B2 - - Google Patents

Description

Detailed Description of the Invention [Industrial Application Field] The present invention relates to a method for measuring optical fiber characteristics, and in particular, to measuring the group delay difference between orthogonal polarization modes of an optical fiber having birefringence using a two-beam interferometer. This paper relates to improvements in optical interferometric measurement methods.

[Prior art] () Delay difference between polarization modes of polarization-maintaining single-mode optical fiber Polarization-maintaining single-mode light that can preserve the polarization state of light waves over long distances as an optical fiber with birefringence Fibers are seen as promising for future applications in the field of optical communications that take advantage of their polarization characteristics, as well as for use in various sensors. For example, a polarization-maintaining optical fiber that preserves two orthogonal linear polarizations is
Since horizontal and vertical polarization can be separated during fiber propagation, it can be used as a medium for coherent optical transmission systems, and communication capacity can be doubled by polarization multiplexing. It is also widely used in various sensors such as static gyroscopes and as a medium, and various optical fiber sensors for temperature, pressure, etc. have been devised by utilizing the birefringence of this fiber as a sensor detection part. There is. Furthermore, it is important as an optical fiber that connects to optical integrated circuits.

In these applications, highly accurate measurement of group delay difference characteristics between orthogonal polarization modes is extremely important for designing coherent optical transmission systems or sensors.

() Group delay difference between polarization modes of a single-mode optical fiber One of the fundamental modes of light propagating in a single-mode optical fiber, the lowest mode is on the Poincaré sphere, which is one way to represent polarization. Two orthogonal polarization modes corresponding to an arbitrary contrast are degenerate, but in reality, the degeneracy is broken due to asymmetric transverse stress, etc., and they appear as orthogonal polarization modes. On the other hand, the polarization-maintaining optical fiber described in item () above intentionally preserves the polarization state. Although only the case where the orthogonal polarization mode is linear polarization will be described below, it is easy to generalize to any orthogonal polarization mode such as right-handed circularly polarized light and left-handed circularly polarized light.

Due to asymmetric transverse stress from internal or external sources,
When there is an effective refractive index difference between orthogonal principal axes in the fiber, a phase difference occurs between orthogonal polarization modes propagating in each principal axis plane. By measuring this phase difference as a group delay difference between orthogonal polarization modes, the characteristics of the optical fiber can be measured. This difference in group delay between orthogonal polarization modes due to the birefringence of the optical fiber is called polarization mode dispersion in the communications field.
This becomes a factor that distorts the received code with respect to the transmitted code of the communication and deteriorates the communication performance. Therefore, measurement of the group delay difference between orthogonal polarization modes is important in clarifying the transmission characteristics of an optical fiber, and is particularly important in designing a coherent optical transmission system.

() Measurement principle using interferometry A method using a two-beam interferometer (also called a two-path interferometer) is a well-known method for measuring the group delay difference between two orthogonal polarization modes propagating in an optical fiber, and is highly accurate. It has features such as being applicable to short optical fibers. An example will be described below in which an interferometer is disposed after the output of the optical fiber, but measurements can also be made using the same principle when the optical fiber is disposed after the interferometer. The orthogonal two-wavelength mode having the group delay difference τ is separated into two optical paths by a polarization separation element of a two-beam interferometer that separates the polarization plane after being emitted from the fiber. For example, as shown in FIG.
It consists of Mv and an optical coupler LC, and its interferometer output is guided to a detector PD to obtain an electronic detection output. Interferometer light splitter P from the same light source LS
If the light that is incident on the detector PD and is separated into two optical paths is detected at a light intensity of ₁ and ₂ respectively at the detector PD at the observation surface position, then the intensity of the light superimposed at the x point on the detector PD is is expressed by the following equation (1).

= ₁ + ₂ + 2γ(τ)√ _{1 2} cosx(1) Here, γ(τ) is the degree of coherence representing the temporal coherence of the light source, and τ is the propagation time difference (group delay difference) between the two light beams. be. The degree of interference is defined by the visibility V of interference fringes on the observation plane, and is expressed by the following equation (2).

V=max-min/max+min (2) where max is the maximum light intensity of the interference fringe,
min is the minimum light intensity of the interference fringe. Above formulas (1), (2)
From this, the following equation (3) can be found.

When the optical system is adjusted so that V=2γ(τ)√ _{1 2} Q/ ₁ + ₂ (3) ₁ = ₂ , V=γ(τ) from equation (3). That is,
When the intensities ₁ and ₂ of the two optical paths are equal, the sharpness is equal to the degree of coherence. Furthermore, when the optical path length difference is zero, the degree of coherence is maximum, and the following equation (4) is obtained.

V=γ(0)=1 (4) By measuring the visibility V using an interferometer in this manner, zero optical path length difference can be determined.

Now, as shown in Fig. 8, two beams of light emitted from the same light source are incident on the interferometer with the group delay difference τ and the refractive index n of the optical path medium mentioned above, and are divided into optical paths and optical paths by a beam splitter. Then, as shown in the figure, an optical path length difference of 2nd is given by a variable optical path length difference imparting device Mν to cancel out the group delay difference, and when the clarity is maximized, assuming the speed of light is c, the following equation is obtained.
The relationship (5) holds true.

τ=2nd/c (5) Therefore, the group delay difference τ can be determined from the optical path length difference 2nd of the interferometer using equation (5).

() Measurement procedure of conventional interference measurement method Conventional interference measurement method using the above-mentioned measurement principle involves reference point measurement (for example, Mochizuki et al., 1981 IEICE Comprehensive National Conference Preliminary Lecture Collection p4- 163), and by cutback measurement (for example, Nobu Shibata, Kogaku Vol. 13, No. 5 (October 1984), p. 383 (25) to p. 387 (29)). Next, measurement procedures using these conventional methods will be explained.

In the case of reference point measurement In this case, the following procedure is performed using, for example, the configuration of a modified Twyman-Green interferometer as shown in FIG.

First, the light emitted from the light source LS is directly input to the interferometer without the fiber F to be measured. A rotation prism P used as a polarization separation element, which is a light splitter of the interferometer, adjusts the plane of incident polarization so that the beam can be separated into two beams at the same time, and adjusts the optical axis of the interferometer.

Next, move the movable mirror Mν to detect the detector PD
The movable mirror position l ₀ at which the interference fringe visibility V is maximum is determined at , and this position is used as the reference point.

Insert the fiber F to be measured into the test equipment.

Find out the main axis direction of the input end and output end of the fiber F to be measured.

Adjustment is made so that the incident light is uniformly incident on the two main axes of the fiber.

The fiber output end angle is set so that the two orthogonal polarized waves emitted from the two principal axes of the fiber F are separately separated by the polarization separation element P into two light beams of the interferometer.

At this time, since the propagation speeds of the light waves propagating through the two principal axes of the fiber F are different, a time difference occurs between the two light waves separated by the polarization separation element at the output end of the optical fiber, and the interference fringe clarity is at its maximum value. I get tired of it. Therefore, the same operation as described above is carried out to find the movable mirror position l at which the interference fringe clarity is maximized.

Since 2d=|l−l ₀ |, the group delay difference τ is determined by the above equation (5).

When using cutback measurement In this case, using the same configuration shown in Figure 9,
Do as follows.

Perform the steps in the above sections in order.

Next, move the movable mirror Mν to detect the detector PD
The movable position l at which the interference fringe clarity is maximized is determined.

The fiber F is removed, cut into short pieces (unit length, usually about 1 m), and reinserted between the polarizer PL of the test device and the rotation prism P as a polarization separation element.

Again, perform the steps in the above section in order.

Perform the operation in this section again to find the movable mirror position l ₂ at which the interference fringe clarity V is maximized. At this time, if the movable mirror position at which the interference fringe clarity is maximized when the fiber output end is rotated by 90 degrees is closer to _l1 , this position is set as _l2 .

From the above l ₂ and l ₁ and each fiber length,
The movable mirror position l ₀ at which the interference fringe clarity is maximized when there is no fiber F is determined by calculation. Since 2d=|l−l ₀ |, the above formula (5)
The group delay difference τ can be found from

[Problems to be Solved by the Invention] However, such conventional measurement methods have the drawbacks that the operating procedure is complicated and adjustment is difficult, as described below.

() Using reference point measurement Before inserting the optical fiber into the test equipment, it is necessary to measure the movable mirror position l ₀ of the interferometer where the optical path length difference is zero. Furthermore, it is necessary to insert the optical fiber into a test device whose optical axis has already been adjusted, and to align the input and output ends of the optical fiber with the optical axis of the test device.

() Cutback measurement This measurement method has the disadvantage that it is a destructive measurement that cuts the fiber under test. Furthermore, it is necessary to once remove the optical fiber from the test apparatus, cut it into a shorter length, and reinsert it into the test apparatus, and each time it is necessary to align the input and output ends of the optical fiber with the optical axis of the test apparatus.

In addition, common to () and () above, due to the complexity of the operation and the difficulty of adjustment, errors may occur due to changes in the optical path in the interferometer due to optical axis deviation during measurement. Therefore, it was difficult to measure with high accuracy.

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for measuring optical fiber characteristics, which eliminates the above-mentioned drawbacks, makes the measurement procedure and adjustment simple and easy, and allows high measurement accuracy to be easily obtained.

[Means for Solving the Problems] In order to achieve the present object, the present invention provides a polarization-separated two-light beam system that provides a propagation time difference between propagating light beams by giving an optical path length difference between one optical path and the other optical path. In a method for measuring the group delay difference between orthogonal polarization modes of an optical fiber using an interferometer and incorporating an optical fiber to be measured on the incident light side or the output light side of a two-beam interferometer,
When the optical fiber to be measured is on the incident light side of the two-beam interferometer, the optical fiber to be measured is placed so that the two orthogonal polarization modes propagating in the optical fiber to be measured are simultaneously and almost uniformly included in the two optical paths of the two-beam interferometer. If the output end of the measurement optical fiber is fixed and the fiber to be measured is on the output side of the two-beam interferometer, the orthogonal polarization 2 of the two-beam interferometer
By fixing the input end of the optical fiber to be measured so that the light flux is contained simultaneously and substantially uniformly in two orthogonal polarization modes of the optical fiber to be measured,
It is characterized by measuring the delay difference between orthogonal polarization modes of the optical fiber to be measured. Here, the orthogonal polarization modes propagating in the optical fiber refer to two orthogonal polarization modes corresponding to arbitrary symmetry points on the Poincaré sphere, which is one method of displaying polarization. However, although only the case where the orthogonal polarization mode is orthogonal polarization will be described below, generalization is easily possible, for example, when the orthogonal polarization mode is clockwise circular polarization and left-handed circular polarization.

[Function] The present invention provides a polarization splitting two-beam interferometer that provides a propagation time difference between propagating lights by giving an optical path length difference between one optical path and the other optical path, and a polarization splitting two-beam interferometer that provides a propagation time difference between propagating lights with respect to one optical path, and a In the installed optical fiber to be measured, two orthogonal polarization modes propagating in the optical fiber are simultaneously included almost uniformly in the two optical paths of the interferometer, or two orthogonal polarization beams of the interferometer are included in the optical fiber. By fixing the optical fiber under test so that the two orthogonal polarization modes are included simultaneously and almost uniformly, the group delay difference between the orthogonal polarization modes of the optical fiber is measured. Adjustment of axis alignment is significantly simplified,
The possibility of errors caused by optical axis misalignment in measurement has been reduced. Furthermore, there is no need to cut the optical fiber to be measured.

[Example] Hereinafter, an example of the present invention will be described in detail with reference to the drawings.

() Basic Embodiment A Configuration FIG. 1 shows a basic configuration example of an optical system of a test device (measuring device) as an example of the present invention.
As the light source LS shown in this figure, for example, a mercury lamp, a light emitting diode, a combination of a white light source and a spectroscope, a semiconductor laser with a short coherence distance, etc. are used. As the fiber F to be measured, two orthogonal
Consider a polarization-maintaining optical fiber that maintains two linearly polarized waves. As the polarization separation element P, which is a light splitter, a rotation prism or the like is used, which separates the incident light beam coming from the fiber F to be measured into two orthogonal polarization states specific to the polarization separation element P. λ/2 is a half wavelength plate, and fixed mirror M
gives a 1/2 wavelength phase rotation to the linearly polarized light reflected by the HM is a light beam passing through a half-wave plate λ/2 and a movable mirror as a variable optical path length adder.
This is a half mirror that serves as an optical coupler that combines the light beam reflected by Mν. The clarity of the interference fringes of the two beams combined by the half mirror HM is determined by the detector
It is detected by a PD, and a photodiode or the like is used as this detector PD.

In the above configuration, the incident light from the light source LS is uniformly incident on the two principal axes of the fiber F to be measured, and uniformly excites the two orthogonal polarization modes. The light emitted from the fiber F is separated into two optical paths Path1 and Path2 by polarization plane by a polarization separation element P.
At this time, the two principal axes (fast axis F and slow axis S) of the output end of the fiber F to be measured are connected to the two separation polarization planes (x-plane and y plane) so that it forms an angle of approximately 45°. As a result, for example, the light in the polarization mode propagating in the fast axis of the fiber under test is polarized with almost equal intensity to the polarization separation planes x and y of the polarization separation element P, and the light is polarized by the interferometer. Proceed through different optical paths, namely Path1 and Path2. At this time,
A group delay difference occurs between two orthogonal polarization modes that have traveled along the fast axis and slow axis of the fiber to be measured. The light on one optical path Path1 passes through the half mirror HM, is reflected by the movable mirror Mν, and is reflected by the half mirror HM.
It reaches the HM, is reflected, and reaches the photodetector PD. The light on the other optical path Path2 is reflected by a fixed mirror M, and the polarization plane is changed by a half-wave plate λ/2.
It is rotated by 90 degrees to have the same polarization plane as the light in the optical path Path1, passes through the half mirror HM, and enters the optical path.
The light enters the photodetector PD through the same optical path as Path1.

The two lights that entered the photodetector PD and passed through Path1 and Path2 have the same polarization state, so they can interfere with each other. That is, by changing the position of the movable mirror Mν to give a difference in the optical path length of the two optical paths, the orthogonal two beams emitted from the optical fiber under test are
The interference fringe clarity reaches a maximum when the group delay difference between polarization modes is cancelled, and the movable mirror exhibits this maximum value.
The group delay difference can be obtained from l ₁ and l ₂ of Mv using the above equation (5).

B. Procedure The procedure according to the method of the present invention when performing measurements using the test apparatus with the optical system shown in FIG. 1 is as follows.

Insert the fiber F to be measured between the light source LS and the polarization separation element P of the test equipment.

Find out the main axis direction of the input end and output end of the fiber F to be measured.

Adjustment is made so that the incident light from the light source LS is uniformly incident on the two principal axes of the fiber F.

The main axis angle of the output end of the fiber F is fixed so as to form an angle of about 45° with the two separation polarization planes of the polarization separation element P.

The movable mirror Mν is moved to determine the movable mirror positions l ₁ and I ₂ at which the interference fringe clarity becomes maximum at the detector PD.

2d=|I ₂ −I ₁ |, and the group delay difference τ is determined by equation (5).

Figure 2 shows an example of measuring the interference fringe definition and 2d value obtained from the detector PD output.

C. Effect As described above, according to this example, measurement is significantly simplified compared to measurement using the conventional interferometry. Specifically, in the section on reference point measurement in the conventional technology mentioned above, the fiber adjustment method can be applied to the test equipment after the optical axis adjustment procedure without introducing errors in the optical path difference of the interference system. In the section on the operation of aligning the optical axes of the input and output ends, and the cutback measurement of conventional technology,
, are not necessary in this example.

That is, according to this example, once the fiber F to be measured is installed in the test device and the optical system is adjusted, the movable mirror Mν is moved to the moving position I ₁ where the interference fringe clarity is maximum, By measuring _I2 , there is no need to make any fine adjustments to the position or direction of the interferometer optical elements that are inevitably used when installing the optical fiber into the test equipment during the measurement process in the conventional method, and the group delay difference can be measured. Since τ is determined by the simple arithmetic expression (5), the optical system only needs to be adjusted once, making measurement very simple. In addition, in this example, the output end angle of the fiber F to be measured does not necessarily need to be adjusted to a strictly 45° angle with respect to the polarization separation element P, and the movable mirror that provides the maximum value is
Since we only need to find the positions I ₁ and I ₂ of Mν,
Adjustment is easy. In addition, the simplification of these procedures and the mutual alignment of the optical axes of the fiber under test F and the interferometer significantly reduces the possibility that an error will be introduced into the interferometer optical path difference due to optical axis misalignment during measurement. Allows to measure group delay differences with precision. Furthermore, unlike the cutback method, it is possible to measure without cutting the fiber under test.

D Modification Figure 3 shows the optical system in Figure 1 with aperture A and lens L.
1, L2 and calculator C and X-Y plotter XYP
An example in which . In this example, as shown in the figure, an aperture (for example, a pinhole, etc.) A is provided in front of the interference fringe detector PD to increase the resolution of the detector PD. Further, condensing lenses L1 and L2 are arranged at the input and output ends of the fiber F to be measured. Calculator C calculates the sharpness using equation (2) from the output from detector PD, and X-Y plotter XYP uses the output of the position detection means (not shown) of movable mirror Mν as an X-axis input signal and calculates Using the sharpness output from the device as a Y-axis input signal, a diagram as shown in FIG. 2 is output on recording paper. Therefore, if the movable mirror Mν is moved in a fixed direction by a driving means (not shown), the X-Y plotter XYP
Measurement data as shown in FIG. 2 can be automatically obtained. It is also possible to simultaneously calculate data such as the group delay difference τ using a calculator C using a microcomputer or the like.

() Other Embodiments FIGS. 4 to 6 each show other configuration examples of the optical system of a test device implementing the present invention. The ones in Figures 4 and 5 use a Michelson interferometer,
The one in FIG. 6 is constructed using a modified Twyman-Green interferometer. Here, p is a beam splitter as a polarization separation element, and λ/4 is 1/4
Wave plate, AL is analyzer.

Although the arrangement order of the fiber F to be measured and the interference system is different from that in Figures 4, 5, and 6, the polarized waves of each of the two orthogonal polarization modes simultaneously pass through both of the two optical paths of the interferometer. They are essentially the same in the sense that they do. In Figure 4, the fiber F
Principal axis direction and polarization separation element PBS at the output end of
The polarization separation plane is fixed at approximately 45°. Fifth
In Figures and Figure 6, the polarization separation element PBS
Alternatively, the principal axis direction at the polarization separation plane by P and the input end of the fiber F is fixed at about 45°. Also,
In Figures 4, 5, and 6, an analyzer AL is installed in front of the interference fringe detector PD, and the transmitted polarization plane of the analyzer AL and the orthogonal two polarization planes of the output light from the interferometer are 45 The interference fringes of the two polarized waves of the emitted light were obtained.

Since the measurement procedure is almost the same as the embodiment shown in FIGS. 1 and 3, detailed explanation thereof will be omitted.
In addition, various configurations similar to this are possible by combining the order of each element component that constitutes the measuring device.

[Effects of the Invention] As explained above, the present invention provides a polarization splitting two-beam interferometer that provides a propagation time difference between propagating lights by giving an optical path length difference between one optical path and the other optical path, and the In the optical fiber to be measured installed on the input light side or the output light side, two orthogonal polarization modes propagating in the optical fiber are simultaneously included almost uniformly in the two optical paths of the interferometer, or By fixing the optical fiber to be measured so that the two orthogonally polarized beams are simultaneously included almost uniformly as two orthogonal polarization modes of the optical fiber, the group delay difference between the orthogonal polarization modes of the optical fiber can be measured. As a result, the measurement procedure and the adjustment of optical axis alignment are significantly simplified, the possibility that errors due to optical axis deviation etc. will be introduced in measurement is reduced, and the group delay difference can be measured with high accuracy.

[Brief explanation of drawings]

Fig. 1 is a block diagram showing the basic configuration of the optical system of an embodiment of the present invention, Fig. 2 is an output diagram showing an example of measurement results obtained in the embodiment of Fig. 1, and Figs. 3 to 6. 7 is a block diagram showing another embodiment of the present invention, and FIG. 7 is a block diagram showing the principle of measurement by interferometry.
FIG. 8 is a block diagram showing the principle of measurement by interferometry, and FIG. 9 is a block diagram showing the conventional measurement method. LS...Light source, F...Fiber under test, P...
Polarization separation element (Rossion prism or Wollaston prism), PBS...Polarization separation element (polarization beam splitter), M...Fixed mirror, Mν...Movable mirror, λ/2...Half Wave plate, λ/4...4
Half-wave plate, HM...Optical coupling element (half mirror), PL...Polarizer, AL...Analyzer, A...Aperture, PD...Detector, C...Calculator, XYP...X
-YProtuta.

Claims (1)

[Claims] 1. Using a polarization-separating two-beam interferometer that provides a propagation time difference between propagating beams by giving an optical path length difference between one optical path and the other optical path, the incident light of the two-beam interferometer is used. In a method for measuring the group delay difference between orthogonal polarization modes of an optical fiber in which an optical fiber to be measured is installed on the side or the output light side, when the optical fiber to be measured is on the incident light side of the two-beam interferometer, By fixing the output end of the optical fiber to be measured so that the two orthogonal polarization modes propagating in the optical fiber to be measured are simultaneously and almost uniformly included in the two optical paths of the two-beam interferometer, the optical fiber to be measured is fixed. A method for measuring optical fiber characteristics characterized by measuring the group delay difference between orthogonal polarization modes of an optical fiber. 2. In the measurement method according to claim 1, when the optical fiber to be measured is on the output light side of the two-beam interferometer, the two orthogonally polarized light beams of the two-beam interferometer are the light to be measured. Measuring the group delay difference between the orthogonal polarization modes of the optical fiber to be measured by fixing the input end of the optical fiber to be measured so that the two orthogonal polarization modes of the fiber are simultaneously included substantially uniformly. A method for measuring optical fiber properties characterized by: 3. In the measurement method according to claim 1 or 2, the orthogonal polarization modes propagating in the optical fiber to be measured are two linearly polarized waves whose polarization planes form a relative angle of 90°. A method for measuring optical fiber characteristics, characterized in that: 4. In the measurement method according to claim 3, each polarization plane of the light emitted from the two principal axes of the optical fiber to be measured makes an angle of approximately 45° with the two separation wave planes of the polarization separation element. A method for measuring optical fiber characteristics, characterized in that the optical fiber is fixed in place. 5. In the measurement method according to claim 3, the orthogonal polarization plane of the incident light from the polarization splitting two-beam interferometer is approximately parallel to the principal axis of the optical fiber to be measured.
A method for measuring optical fiber properties characterized by fixing the fiber at a 45° angle. 6. In the measurement method according to claim 1 or 2, the first incident light enters one optical path of the two-beam interferometer, and the first incident light has a polarization state orthogonal to the first incident light, An optical fiber characteristic characterized in that at least one polarizing element is disposed in the interferometer optical path so that the light incident on the other optical path has the same polarization state at the output light section of the interferometer. measurement method. 7. In the measurement method according to claim 6, the optical fiber characteristics are characterized in that the orthogonal polarization states are orthogonal linearly polarized waves, and the polarizing element is a half-wave plate. measurement method. 8. In the measurement method according to claim 1 or 2, the light propagated through the optical fiber to be measured and the interferometer is guided to a detector, and the light consists of two orthogonally polarized light beams combined. A method for measuring optical fiber characteristics, which sometimes comprises arranging a polarization element in front of the detector to combine the two beams of orthogonal polarization state with equal amplitude. 9. The method of measuring optical fiber characteristics according to claim 8, wherein the two light beams are orthogonal linearly polarized waves, and the polarization element is an analyzer.